The present invention relates in general to detecting misfires occurring during normal in-use vehicle operation of internal combustion engines and more specifically to identifying the occurrence of individual misfires by digitally processing a pressure wave from the exhaust manifold of the engine.
Catalytic converters are used in automobiles to reduce the amount of pollutants in the engine exhaust. When a cylinder misfires so that no combustion or incomplete combustion occurs, uncombusted fuel is introduced into the exhaust which then burns in the hot catalytic converter. The heat from fuel burning in the catalytic converter destroys the catalyst. Thus, it becomes desirable to detect and count engine misfires and signal the operator of the vehicle upon occurrence of excessive misfires so that steps may be taken to protect the catalytic converter.
Some prior art techniques for detecting misfires have employed monitoring of engine crankshaft accelerations, monitoring of electrical properties of the ignition spark, and monitoring various properties of the exhaust gas, such as, exhaust gas pressure and exhaust gas temperature. However, each prior art method has been found to have disadvantages which have prevented the attainment of an acceptable misfire detection system. Low signal-to-noise ratios and slow operating speed have required averaging of many events in some previous misfire detection systems. Such systems are only capable of detecting recurrent misfires of any particular cylinder rather than individual misfires. Other systems may require expensive custom sensors and components or may require disadvantageous sensor locations. Furthermore, existing misfire detection systems all suffer from poor accuracy which prevents any ability to identify very low misfire rates. For example, in order to protect catalytic converters and prevent excessive emissions of pollutants, a misfire rate of about one or two percent must be detected. In addition, the identity of the misfiring cylinder associated with each individual misfire must be determined and stored to facilitate later servicing of an engine to correct the condition leading to the misfires. Typically, such diagnostic strategies must have very low false alarm rates in order to be deployed in large numbers of vehicles.